The present invention relates to a microwave oven with toasting and baking functions having an upper and lower heater in the heating chamber, or particularly to a microwave oven having a circuit for periodically energizing and de-energizing one of the two heaters.
Typically, a microwave oven with toasting and baking functions comprises the circuitry shown in FIG. 4, and controls, according to the selected cooking mode, the power supply to upper and lower heating elements (50) and (51) in the heating chamber (not shown) and to high frequency heating means (56) comprising a magnetron (52), a high voltage transformer (53), a high voltage capacitor (54) and a high voltage diode (55). The cooking mode is selected from among toasting, baking, defrosting and high-frequency heating (hereinafter referred to as microwave heating) modes.
The operation of the electrical elements in the baking mode is described with reference to FIG. 4.
When the baking mode is selected, the N.O. contact (57a) of the selector switch (57) is closed. The toasting switch (58) and the defrost-cancelling switch (59) are then closed interlocking with the N.O. contact (57a). To control the temperature in the heating chamber (not shown), the oil thermostat (60) provided in the heating chamber is adjusted to set the heating temperature.
When heating time is set by a timer (61), the timer contact (61a) closes so that power is supplied to the upper and lower heating elements (50) and (51) arranged in the heating chamber. The oil thermostat (60) is turned OFF when the temperature in the heating chamber rises above the setting, and turned ON when the heating temperature drops to the setting, thus maintaining the heating temperature at the preset value. When the preset heating time has elapsed, the timer contact (61a) opens, terminating the cooking operation.
However, in a microwave oven with the above circuitry in which the power supply to the two heaters is turned ON and OFF around the preset heating temperature, the radiant heat is too large for baking, say, cake, often causing cracks in the cake surface.
One possible measure against this is to lower the heating temperature or to keep the heaters away from the food. These measures have the problems that the cooking operation takes longer as the heating temperature is lowered, and the remoter the heaters are from the food, the longer it takes in toasting food, sacrificing the food taste.